Conventional rear projection screens use a thin diffusing layer in the form of a matte screen. An image is projected onto the back of the diffusing layer, where it is diffused and scattered into the viewing environment. The diffusing layer provides an image surface and its diffusive nature serves to increase the viewing angles from which the image can be observed. Often these simple diffusing layer projection screens have poor contrast in well-lit rooms. This poor contrast is due, at least in part, from the scattering of ambient light back into the viewing environment, thereby deleteriously affecting the black levels and contrast of the rear projection screen.
FIGS. 1A and 1B illustrate a conventional rear projection screen 50 that uses small glass spheres 62 embedded in a black adhesive 61 on a flat glass plate 60. From the top side, the majority of the screen surface is covered by black adhesive 61, thereby reducing the back reflections of ambient light and improving display screen contrast. Each glass sphere 62 forms a thin channel through black adhesive 61 and focuses incident light through a pinhole 66 that is self-aligned to a corresponding glass sphere 62. Glass spheres 62 gather the majority of the backside incident light and focus it through pinholes 66. FIG. 1B illustrates a scenario where on-axis light 64 that is perpendicular to rear projection screen 50 is focused by a glass sphere 62 through pinhole 66. However, off-axis light 65 (illustrated in FIG. 1A) must be bent using a Fresnel lens 63. Fresnel lens 63 operates to receive obliquely incident light and bend it such that the light is incident through glass spheres 62 at near normal angles to glass plate 60. The off-axis light 65 must be bent to a normal trajectory to maintain angular brightness uniformity across the front side of the screen. This design operates to maintain screen output efficiency while increasing the screen contrast in well-lit rooms. However, it requires that backside incident light be incident at a near normal angle to achieve acceptable angular brightness uniformity and uses a Fresnel lens 63 to do so.
FIGS. 1C and 1D illustrate another conventional rear projection screen 75. Rear projection screen 75 operates in a similar manner to rear projection screen 50. Rear projection screen 75 includes an array of lenslets 83 replicated across the back surface of a substrate 82 having a layer of dark material 81 patterned across the top surface. Dark material 81 is patterned using a high-powered laser directed at lenslets 83. Lenslets 83 focus the beam onto dark material 81 burning or ablating away pinholes that are self-aligned to lenslets 83. Similar to rear projection screen 50, incident light must be directed onto lenslets 83 along a direction that is substantially perpendicular or normal to the surface of substrate 82. Again, normal incidence is achieved using a Fresnel lens 84. This design also operates to maintain screen output efficiency while increasing the screen contrast in well-lit rooms. However, as before it requires that backside incident light be incident at near normal angles to achieve acceptable angular brightness uniformity and uses a Fresnel lens 84 to do so.